1. Field of the Invention
This invention relates to the controlled release of volatilized liquids. More particularly, this invention relates to a container, method and permeable materials for controlling the release of at least one volatilize ingredient, such as a fragrance or a biologically active material, from an aqueous mixture of ingredients confined within the container into the adjacent atmosphere.
2. Description of the Prior Art
Containers for volatile materials, including fragrances, air fresheners and biologically active compounds such as pheromones have been designed to control the rate at which these materials are released through at least one permeable section of the container into the atmosphere surrounding the container. The containers confine a liquid or solid composition that includes the volatile material.
Depending upon the type of volatile material and the desired release rate, the portion of the container that is permeable to the volatile material can be a layer of non-porous material such as silicon rubber, a porous film or layer of an organic polymer or a coating of an organic polymer on a porous substrate such as paper. The volatile material readily permeates through both the substrate and the coating. Containers of various configurations, both rigid and flexible, are commercially available and/or are disclosed in patents and other literature.
The exterior surface of the permeable portion of a container for dispensing a volatile material can be equipped with a temporary cover that is impermeable to the volatilized material and prevents loss of this material between the time the container is filled with the material and the time it is desired to begin releasing the material.
A variety of organic and inorganic materials have been proposed for use as the permeable portion of containers for releasing volatile materials into a gaseous environment. In accordance with the teaching of U.S. Pat. No. 4,158,440, which issued to Sullivan et al. on June 19, 1979 the volatile material is absorbed into a reservoir material that is in contact with a sheet of ultramicroporous material such as gelled cellulose triacetate into which the volatile liquid is gradually absorbed and released from the container.
The use of thin polymeric membranes having pore diameters of from 10.sup.-3 to 10.sup.-1 microns to control the release of vapors from a non-flowing liquid confined in a reservoir is taught in U.S. Pat. No. 4,356,969, which issued to Obermayer et al. on Nov. 2, 1982. The material of the membrane is one that will preferentially absorb the volatile liquid and the diameter of the pores is sufficient to allow the volatile liquid to pass through and vaporize from the pores without allowing non-vaporized liquid to arrive at the outer surface of the membrane. To release an organic liquid fragrance the membrane is preferably formed from an organophillic polymer such as cellulose triacetate, polyethylene, polypropylene or a composite of one of these polymers and a liquid.
U.S. Pat. No. 4,600,146 to Ohno, which issued on July 15, 1986 teaches confining a vaporizable liquid in a capillary tube formed from a polymeric material that is permeable to the vaporized or unvaporized liquid. The polymeric material can be an organic polymer or an organopolysiloxane.
U.S. Pat. No. 4,605,165, which issued to Van Loveran et al. on August 12, 1986 discloses containers for dispensing a volatile composition at a constant rate. The composition is allowed to permeate through a layer of porous polymer that is from 0.0025 to 0.5 mm thick. The composition of the polymer layer is defined in terms of its ability to (a) transport water vapor at a rate of between about 50 and 1000 g/m.sup.2 /day at a temperature of about 25 C. and a relative humidity of about 50%, or (b) transport air at a rate of 100-20,000 Gurley seconds. The only exemplified polymer is polypropylene containing a calcium carbonate filler.
Published European Application No. 218,891, which issued on Apr. 22, 1987 discloses a container for dispensing a volatile substance into the atmosphere adjacent to the container. The container is formed from silicone rubber and the substance diffuses through the silicon rubber into the environment surrounding the container. The high permeability of silicone rubber to gasses is allegedly responsible for the high rates of release that can be achieved. The application does not contain examples or data disclosing the rate at which any specific material is released.
Published European Application No. 218,892, which issued on Apr. 22, 1987, describes a dispenser for releasing a volatile liquid. The container comprises an open-ended hollow body formed from a material that is impermeable to the volatile liquid and a closure for the container that is in contact with the volatile liquid. The closure is formed from a silicone rubber and is permeable to the volatile liquid. No experimental data regarding release rates are provided.
For some end use applications it is advantageous to incorporate a volatile fragrance, air freshener or biologically active compound such as a bactericide, herbicide or the sex pheromone of an insect pest into an aqueous mixture from which the liquid is subsequently volatilized and released into the atmosphere. In this instance the release rate of the volatile liquid is proportional to its partial pressure in the mixture and the permeability of the material separating the aqueous mixture from the atmosphere.
Controlled release into the atmosphere of volatile liquids from an aqueous mixture has been achieved by confining the mixture in a container wherein the composition is in contact with the inner surface of a layer of a porous foam formed from organic polymer. The outer surface of the foam is exposed to the environment into which the volatilized liquid is to be released.
The disadvantage of such an container is that if the foam is sufficiently porous to achieve the desired release rate of the volatile material, which can be from 0.1 to 100 mg/(cm.sup.2 .times.day), depending upon the nature of the material being released, the foam layer also permits the other less volatile ingredients of the aqueous mixture, to permeate the foam layer at a rate exceeding their evaporation rate, thereby allowing these ingredients to accumulate on the exposed surface of the foam layer.
One solution to this problem is to locate the porous foam layer out of direct contact with a reservoir containing the aqueous mixture. The mixture is allowed to diffuse to the foam layer through a wick of absorbent material. A disadvantage of such an arrangement is that the exposed surface of the foam can be wet if the rate at which the aqueous composition permeates through the foam is greater than the evaporation rate of the less volatile ingredients, particularly water. A more serious disadvantage is that when the container is placed in a position that allows the aqueous mixture to flow out of the reservoir and contact the foam layer, the mixture typically leaks through the foam and out of the container.
The problems of liquid accumulation and leakage can be avoided by covering or replacing the porous foam layer with a film or coating of a material that is impervious to the aqueous mixture while allowing vaporized liquids from the mixture to penetrate and be released into the atmosphere adjacent to the film or coating. For many volatile materials it is desirable that the rate at which the liquids are released through the film or coating be at least about 70 percent of the release rate through the uncovered porous polymer layer and that the exposed surface of the film or coating remain dry during the entire release period. The present inventors endeavored to find materials that satisfy these two criteria.
Membranes of silicone rubber such as those disclosed in the aforementioned published European Patent Applications were insufficiently permeable to provide the desired rapid release rate of a volatilized fragrance from an aqueous mixtures Films and coatings formed from the hydrophobic organic polymers disclosed in the aforementioned U.S. Pat. No. 4,605,165 to Van Loveran et al. were unsuitable for the same reason.
Films and coatings formed from hydrophilic polymers such as polyoxyethylene glycols release the volatile ingredients of aqueous mixtures considerably more rapidly than silicone rubber or hydrophobic organic polymers. The present inventors found these hydrophilic polymers to have the same disadvantage, namely leakage of the aqueous mixture, as the porous hydrophobic polymers they were intended to coat or replace, resulting in accumulation of liquid on the exposed surface of the film or coating. In addition these hydrophilic polymers are susceptible to solubilization and/or excessive swelling by the aqueous mixture, resulting in frequent rupturing of the film or coating.
U.S. Pat. No. 4,686,137, which issued to Ward and Riffle on Aug. 11, 1987, discloses moisture vapor permeable block copolymers consisting essentially of a "hard" segment that is preferably a polyurethane formed from the reaction of diphenylmethane diisocyanate with a diol. and a "soft" segment having both hydrophilic and hydrophobic portions. The hydrophobic portion of the soft segment can be a polymeric tetraalkylene oxide, such as polytetramethylene oxide, a polydialkylsiloxane, or a mixture of these two polymers, and the preferred hydrophilic segment is polyethylene oxide. These copolymers are combined with a base polymer, such as a polyurethane, and a suitable solvent to form films suitable for use as wound dressings or semipermeable membranes and as coating compositions for textile materials.
An objective of this invention is to provide materials for films or coatings that are inert to aqueous mixtures while allowing volatile ingredients of these mixtures to migrate through the film or coating and be released into an adjacent gaseous environment at a suitably rapid rate without the accumulation of liquid material on the exposed surface of the film or coating. These films or coatings can be used alone or in combination with a layer of porous organic or inorganic material to control the release rate of the volatile ingredients.